1. Field of the Invention
The present invention relates, in general, to solenoid valves for brake systems of automobiles and, more particularly, to a solenoid valve used in the oil return line of an electro-hydraulic brake system.
2. Description of the Prior Art
Of a variety of brake systems recently proposed and used in automobiles, an electro-hydraulic brake system (EHB) is designed such that it senses pedal pressure, applied to the brake pedal by the driver""s foot, through a pressure sensor, and controls oil pressures, which will be fed to the wheel brake cylinders, using a hydraulic modulator in response to the sensed pedal pressure.
As shown in FIG. 1, a conventional EHB comprises a pedal operation sensor 2, which senses the operation of a brake pedal 1. A master cylinder 4 is connected to the pedal 1, and has a pressure sensor 3 used for sensing pedal pressure applied to the pedal 1 by the driver""s foot. The EHB also has an oil pump 6, which forcibly sucks oil from an oil tank 5 and outputs the oil under pressure. A accumulator 7 receives and stores the pressurized oil outputted from the pump 6. The EHB further includes an inflow control solenoid valve 10 and an outflow control solenoid valve 20. The inflow control solenoid valve 10 is mounted on an oil supply line extending from the accumulator 7 to a wheel brake cylinder 8, and controls a supplying of pressurized oil from the accumulator 7 to the brake cylinder 8. The outflow control solenoid valve 20 is mounted on an oil return line extending from the wheel brake cylinder 8 to the oil tank 5, and controls a returning of oil from the cylinder 8 to the tank 5. The EHB also has an emergency oil line 9, which directly feeds pressurized oil from the master cylinder 4 to the brake cylinder 8 and brakes the wheels to provide against emergencies, such as malfunction of the brake system. A solenoid valve 9a is mounted on the emergency oil line 9 to control the line 9.
The above conventional EHB is operated as follows: That is, when pedal pressure is sensed by the pressure sensor 3 of the master cylinder 4, an electronic control unit (ECU, not shown) opens the inflow control solenoid valve 10 in response to the pedal pressure, thus allowing pressurized oil to be fed from the accumulator 7 to the brake cylinder 8 and forming braking pressure in the cylinder 8. The accumulator 7 normally stores pressurized oil outputted from the oil pump 6, and maintains preset oil pressure. The operation of the oil pump 6 is controlled in response to a signal outputted from a pressure sensor 7a provided at the outlet of the accumulator 7. That is, the pressure sensor 7a senses oil pressure inside the accumulator 7, and outputs a signal to the ECU to selectively operate the pump 6.
At a time of removing the braking pressure from the brake cylinder 8, the inflow control solenoid valve 10 is closed, but the outflow control solenoid valve 20 is opened under the control of the ECU. Pressurized oil is thus returned from the brake cylinder 8 to the oil tank 5 provided at the master cylinder 4. During such an operation of the EHB, the inflow and outflow control solenoid valves 10 and 20 are alternately and repeatedly opened and dosed under the control of the ECU, thus repeatedly and intermittently applying braking pressure to the brake cylinder 8 in the same manner as a conventional anti-lock brake system (ABS). The EHB thus prevents a slippage of the wheels on the road during a braking operation. In such conventional EHBs, a normal dose-type solenoid valve (hereinbelow, referred to simply as xe2x80x9cNC-type solenoid valvexe2x80x9d) is typically used as each of the inflow and outflow control solenoid valves 10 and 20. Such NC-type solenoid valves normally maintain their dosed states, and are preferably used as the solenoid valves 10 and 20 of such an EHB in consideration of desired operational characteristics and operational efficiency of the solenoid valves 10 and 20.
FIG. 2 is a sectional view showing the construction of the inflow and outflow control solenoid valves 10 and 20 set in a modulator block of the conventional EHB. As shown in the drawing, the modulator block 30 of the EHB seats the inflow and outflow control solenoid valves 10 and 20 therein, and has a plurality of complex oil passages. When designing the modulator block 30, it is necessary to consider such complex oil passages, in addition to work efficiency while machining the block 30 and seating the valves 10 and 20 in the block 30, as well as the recent trend of compactness and smallness of the modulator blocks. The inflow and outflow control solenoid valves 10 and 20 are parallely installed in the modulator block 30 in consideration of such structural characteristics of the block 30. That is, the inflow and outflow control solenoid valves 10 and 20 are parallely and vertically installed in the modulator block 30 at left- and right-hand sides of FIG. 1, respectively.
During the operation of the EHB, pressurized oil outputted from the accumulator 7 is introduced into the inflow control solenoid valve 10 through an inlet oil passage 31 formed in the block 30 at a position under the valve 10. Thereafter, the inlet pressurized oil laterally flows from the inflow control solenoid valve 10 into a connecting passage 32 which communicates the two valves 10 and 20 to each other. The connecting passage 32 is also connected to another oil passage 33, which is connected to the wheel brake cylinder 8. Therefore, when the inflow control solenoid valve 10 is opened, pressurized oil outputted from the accumulator 7 is fed to the brake cylinder 8, thus forming braking pressure in the cylinder 8 and braking the wheels.
At a time of removing the braking pressure from the brake cylinder 8, the inflow control solenoid valve 10 is closed, but the outflow control solenoid valve 20 is opened under the control of the ECU. Pressurized oil is thus outputted from the brake cylinder 8, and flows into the outflow control solenoid valve 20 through a radial oil passage 21 of the valve 20, and is returned to the oil tank 5 of the master cylinder 4 through an outlet oil passage 34 formed in the block 30 at a position under the valve 20.
As described above, the inflow and outflow control solenoid valves 10 and 20 are NC-type solenoid valves. The operation of the inflow control valve 10 is easily controlled, but the control of the outflow control valve 20 is difficult due to the relation between the structural characteristics of the NC-type solenoid valves and the flowing direction of oil during the operation of EHB.
In a detailed description, when the plunger 11 axially moves upward to open the orifice 12 in the inflow control solenoid valve 10, pressurized oil from the inlet oil passage 31 passes upward through the open orifice 12 to flow into the radial oil passage 13 of the valve 10. In such a case, a large pressure difference is formed between the inlet and outlet of the orifice 12 at the initial stage of opening the orifice 12, and so oil pressure inside the orifice 12 acts in an upward direction wherein the plunger 11 moves to open the orifice 12. The plunger 11 is thus biased upward by the oil pressure. After a predetermined lengthy period of time, passes, the plunger 11 sufficiently opens the orifice 12, and so the pressure difference between the inlet and outlet of the orifice 12 is reduced to a predetermined level. In such a case, the flow rate of oil through the orifice 12 is increased, but the oil pressure inside the orifice 12 is reduced. The plunger 11 thus has a tendency to close the orifice 12. The inflow control solenoid valve 10 thus provides a xe2x80x9cself-equilibrating effectxe2x80x9d. Due to such a self-equilibrating effect, it is easy to control the operation of the inflow control solenoid valve 10, wherein the oil flows upward from the bottom of the valve 10 to the radial oil passage 13. However, in the outflow control solenoid valve 20, oil flows downward from the radial oil passage 21 into the outlet oil passage 34. That is, the oil inside the outflow control valve 20 flows in a reverse direction to that of the inflow control valve 10, and so such a self-equilibrating effect is not rendered in the outflow control valve 20 even though the valve 20 has the same construction as that of the inflow control valve 10. It is thus difficult to control the operation of the outflow control solenoid valve 20. Such a problem experienced in the control of the operation of the outflow flow control solenoid valve 20 may be overcome by changing the oil passage structure in side the modulator block 30 such that the oil structure of the outflow control valve 20 becomes the same as that of the inflow control valve 10. However, it is almost impossible to practically design the oil passage structure of the block 30 to accomplish the above object, because such a change in the oil passage structure of the modulator block runs counter to the recent trend of compactness of the modular blocks.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a solenoid valve for brake systems, which is improved in its oil passage structure to render a self-equilibrating effect therein even though oil flows from the radial oil passage formed at the sidewall of the valve to the outlet oil passage formed in the modulator block at a position under the valve, and which is thus easily controlled during the operation of a brake system.
In order to accomplish the above objects, the present invention provides a solenoid valve for brake systems, comprising: a hollow valve housing having an oil inlet passage on its sidewall, with a bore axially defined in the valve housing; a plunger movably received in the bore of the valve housing so as to axially move in the bore in opposite directions by an electric force; a valve seat set in the bore of the valve housing, with an orifice axially formed in the valve seat such that the orifice is opened or dosed by an end of the plunger in accordance with an axial movement of the plunger, a radial oil port formed on the sidewall of the valve seat so as to allow oil from the oil inlet passage of the valve housing to flow into the valve seat through the radial oil port, and flow from the valve seat toward the plunger through the orifice of the valve seat; a plug dosing the lower end of the bore of the valve housing, thus allowing the oil to flow from the valve seat toward the plunger through the orifice; and an oil outlet passage axially formed in the sidewall of the valve housing in parallel to the bore such that the oil outlet passage communicates at its top end with the outlet of the orifice, the oil outlet passage feeding the oil from the outlet of the orifice to an area under the bottom of the valve housing.